feat: dynamic custom frontmatter fields + CTF writeup template system

User requested dynamic (not static) fields for CTF writeup content organization.
Changes:
- DocumentMeta: removed typed CTF fields (event/challenge/category/difficulty/
  points), replaced with single extraFields?: Record<string, string>
- parseFile: any frontmatter key not in the STANDARD set becomes a dynamic
  extra field — fully content-driven, no code changes needed for new fields
- stringifyFile: writes extraFields back to YAML frontmatter for round-trip
  stability
- DocForm: '+ Add field' UI lets creators add ANY metadata key at create/edit
  time
- API routes: splitPayload() auto-separates standard vs custom fields
- Doc page: CustomFieldBadges dynamically renders any custom field with
  auto-styling for common CTF patterns (difficulty→color, points→badge)
- Added db:migrate + db:push scripts; deploy workflow now runs migrations +
  reindexes content on every deploy
- content/writeups/ctf/template/writeup-template.md (template)
- content/writeups/ctf/defcon-quals-2024/pwn-100-ret2win-alignment.md (sample)
- DB column extra_fields (jsonb) already applied to live DB

packages/types/src/index.ts

@@ -14,14 +14,8 @@ export interface DocumentMeta {
   path: string; // relative path under content/
   createdAt: string; // ISO
   updatedAt: string; // ISO
-  // CTF writeup metadata (optional — only set for writeups)
-  event?: string;
-  challenge?: string;
-  category?: string;
-  difficulty?: "easy" | "medium" | "hard";
-  points?: number;
-  // Dynamic custom frontmatter fields (serialized as-is to YAML frontmatter).
-  // Allows content creators to add arbitrary metadata keys without code changes.
+  // Dynamic custom frontmatter fields (e.g. CTF: event, challenge, category,
+  // difficulty, points). Content creators add arbitrary key-value pairs.
   extraFields?: Record<string, string>;
 }